1. Field of the Invention
The present invention relates to a pixel circuit board usable for an active matrix display panel, a test method of the pixel circuit board, a pixel circuit arranged on the pixel circuit board, a test method of the pixel circuit, and a test apparatus.
2. Description of the Related Art
Organic electroluminescent display panels can roughly be classified into passive driving types and active matrix driving types. Organic electroluminescent display panels of active matrix driving type are more excellent than passive driving types because of high contrast and high resolution. In an organic electroluminescent display panel of active matrix display type described in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 8-330600, an organic electroluminescent element (to be referred to as an organic EL element hereinafter), a driving transistor which supplies a current to the organic EL element when a voltage signal with a voltage value corresponding to image data is applied to the gate, and a switching transistor which performs switching to supply the voltage signal corresponding to image data to the gate of the driving transistor are arranged for each pixel. In this organic electroluminescent display panel, when a scan line is selected, the switching transistor connected thereto is turned on. At this time, a voltage of level representing the luminance is applied to the gate of the driving transistor through a signal line. The driving transistor connected to the signal line is turned on. A driving current having a magnitude corresponding to the level of the gate voltage is supplied from the power supply to the organic EL element through the driving transistor. The organic EL element emits light at a luminance corresponding to the magnitude of the current. During the period from the end of scan line selection to the next scan line selection, the level of the gate voltage of the driving transistor is continuously held even after the switching transistor is turned off. Hence, the organic EL element emits light at a luminance corresponding to the magnitude of the driving current corresponding to the voltage.
The manufacturing process of driving transistors and switching transistors includes a step in which the temperature exceeds the heatresistant temperature of organic EL elements. For this reason, in manufacturing an organic electroluminescent display panel, driving transistors and switching transistors are manufactured before organic EL elements. Preferably, driving transistors and switching transistors are patterned on a substrate to prepare a transistor array board first. Then, organic EL elements are patterned on the transistor array board.
In the above-described transistor array board, it is difficult to determine by a test after manufacture of the organic EL elements whether a failure is caused by a transistor or an organic EL element. In a test before the organic EL elements are manufactured, the transistors are not connected to the organic EL elements. Electrodes (one of the source and drain) of the transistors, which should be connected to the organic EL elements, are electrically independent for each pixel and are in the floating state. In testing the transistors on the transistor array board, the electrodes of the transistors, which should be connected to the organic EL elements, may be probed for each pixel. In this case, the test must be done by inefficiently executing probing for each pixel. The other electrodes (the other of the source and drain) of the transistors, which should be connected to the organic EL elements, are connected to the power supply lines. For this reason, the transistors can be read-accessed from the power supply lines. In this case, the electrodes of the driving transistors, which should be connected to the organic EL elements, must be connected to a constant potential line.